Although there are numerous methods of teaching people to speak a different language apart from their mother tongue, there is no one universally accepted theory of how they actually learn that second language. Our philosophy in LinguaTutor is that language is learned not for its own sake but in order to communicate and to find out about the world. For that reason our teaching is organized around diverse and interesting topics, such as ecology, inventions, news, daily events, sports, arts, culture, language, etc. Each topic consists of linguistic tasks, and incorporates various essential communicative skills (listening, speaking, reading and writing). Students are exposed to written and spoken language that is (made) comprehensible to them, and are engaged in different kinds of creative activities to encourage communication in their secondary language.
There is no simple answer to this question. The best way of learning depends very much on the learner and why he/she wants to learn. An extrovert will be probably learn better with a method that involves lots of role plays and participation in unstructured discussions, whereas a quieter, more reflective person may prefer written exercises with plenty of grammatical explanations. Someone who is only interested in being able to communicate well enough on holiday in the foreign country will require a different method of learning than another person who wants to study at a foreign university. For the general language learner an eclectic mix of several different methods is often the best approach.